Add %toolchain macro to differentiate C/C++ toolchains

[tbaederr]

This is often needed when supporting builds with multiple compilers. Simply looking at %__cc is not enough since the actual executable might be called differently, but might still be gcc. Or clang.
Doing a substring search is also problematic since "g++" is a substring of "clang++".

So it would be nice if rpm provided a standard macro for this.

Does this make sense for RPM and does %toolchain sound fine? If so, I can work on the actual implementation.
It does seem like the value of %toolchain would have to be set for every compiler specially though.

[pmatilai]

Not going into implementation and naming here, but the issue is certainly valid and applies broadly to all sorts of things, including but not limited to compiler flags from %optflags.

[voxik]

I wonder if there is any possible relation to https://src.fedoraproject.org/rpms/redhat-rpm-config/pull-request/82

[tbaederr]

We will solve this differently, in redhat-rpm-config. Thanks.